1. Field of the Invention
The invention relates to a discharge lamp which has a discharge vessel of silica glass and which has an operating pressure higher than atmospheric pressure.
2. Description of Related Art
In a discharge lamp with high radiance such as for example, a super-high pressure mercury lamp, a rare gas-mercury lamp, a metal halide lamp, or the like, a discharge vessel of silica glass is used. Here, the expression xe2x80x9csilica glassxe2x80x9d is defined as a material which contains SiO2 with a percentage by weight of greater than or equal to 98% and in which greater than or equal to 90 percent by volume is amorphous.
In the operation of one such discharge lamp with high radiance the temperature of the outer surface of the arc tube portion of the discharge vessel reaches 850 to 1000xc2x0 C. The temperature of the inner surface of the arc tube portion is roughly 50 to 150xc2x0 C. higher than the temperature of the outer surface. Furthermore, in operation of the discharge lamp, the tube wall of the arc tube portion is exposed to a tensile force due to the internal pressure (operating pressure).
Since when the discharge lamp is operating, the vicinity of the inner surface of the arc tube portion reaches a higher temperature (for example, 900 to 1150xc2x0 C.) than the vicinity of the outer surface, the glass in the vicinity of the inner surface has a higher viscous flow property than the glass in the vicinity of the outer surface. As a result thereof, when the discharge lamp is turned off, the tensile force exerted on the vicinity of the inner surface of the arc tube portion is reduced by the viscous flow of the glass. The tensile force exerted on the vicinity of the outer surface of the arc tube portion however remains, by which in the vicinity of this outer surface, a tensile load (hereinafter called xe2x80x9cthermal distortionxe2x80x9d) is formed. The thermal distortion in the vicinity of the outer surface of the arc tube portion becomes greater with increasing length of operation. Since cracking in glass forms on the outer surface on which stress corrosion by water occurs, damage and breaking of the discharge vessel are caused as a result.
Therefore, the primary object of the invention is to devise a discharge lamp in which even after operation over a long time, neither damage nor breaking of the discharge vessel occurs due to thermal distortion.
The object is achieved in accordance with preferred embodiments of the present invention in a discharge lamp as follows:
(1) In one embodiment of a discharge lamp with a discharge vessel of silica glass which has a higher operating pressure than atmospheric pressure, the high temperature viscosity (VA) in the outer surface area of the arc tube portion of the discharge vessel is lower than the high temperature viscosity (VB) of the arc tube portion at a depth of 100 microns from the outer surface.
(2) In another embodiment of a discharge lamp with a discharge vessel of silica glass which has a higher operating pressure than atmospheric pressure, the outer surface area of the arc tube portion of the discharge vessel is subjected to a viscosity reducing treatment.
In the invention, the expression xe2x80x9couter surface areaxe2x80x9d is defined as an area in the direction of the wall thickness of the tube which forms the arc tube portion, specifically, an area with a depth of 20 microns from the outer surface of the arc tube portion.
The expression xe2x80x9chigh temperature viscosityxe2x80x9d is defined as the coefficient of viscosity which is measured under the condition of a constant temperature of greater than or equal to 850xc2x0 C. (for example 1200xc2x0 C.).
The values of the high temperature viscosities (VA) and (VB) to be compared to one another, are values which are determined by measurement under the same temperature condition.
xe2x80x9cHigh temperature viscosity (VB) at a depth of 100 microns from the outer surfacexe2x80x9d is defined as the value of the high temperature viscosity which is determined by measurement of the xe2x80x9cdepth from the outer surfacexe2x80x94curve of high temperature viscosityxe2x80x9d.
Furthermore xe2x80x9cviscosity reducing treatmentxe2x80x9d is defined as a treatment for reducing the high temperature viscosity (formation of a layer with reduced viscosity and diffusion of a material with a reduced viscosity).
In the discharge vessel in accordance with the present invention with one such arrangement, the advantages are the following:
(1) Since the high temperature viscosity (VA) in the outer surface area is lower than the high temperature viscosity (VB) at a depth of 100 microns from the outer surface, the glass in the outer surface area during operation of the discharge lamp has a higher viscous flow property than the glass at a depth of 100 microns from the outer surface. As a result, even after the lamp is turned off, in the outer surface area the tensile force is not easily obtained. Therefor, frequent occurrence of thermal distortion is prevented. In the case in which in the outer surface area thermal distortion does not occur, the strength of the silica glass (discharge vessel) can be adequately guaranteed even if in the interior (in an area from a depth of 100 microns to the inner surface), thermal distortion occurs. Thus, formation of damage and breaking of the discharge vessel can be reliably prevented. The reason for this is that cracking of the glass starts primarily proceeding from the outer surface.
(2) Due to the viscosity reducing treatment of the outer surface area of the arc tube portion, the glass in the outer surface area under the temperature condition during operation of the discharge lamp (850 to 1000xc2x0 C.) has a higher viscous flow property than inside this area.